Discharge lamp with baffle plates

ABSTRACT

A low pressure metal vapor discharge lamp, such as of mercury vapor, is provided with internal interdigitated apertured baffle plates. This lengthens the discharge path and reduces temperature to provide increased efficiency and improved operation at higher intensities. The baffles may be of insulation or conductor materials and may be coated with a protective layer, fluorescent material or ultra-violet reflective layer.

United States Patent [1 1 Taxil et al.

[ 1 Nov. 12, 1974 1 1 DISCHARGE LAMP WITH BAFFLE PLATES [75] lnventors:Andr Marc Victorin Taxil,

Rueil-Malmaison; Raymond Claude Emile Boucher, La Garenne Colombes, bothof France [73] Assignee: ITT Industries, Inc., New York,

221 Filed: Mar. 14, 1973 21 Appl. No.: 341,004

[52] US. Cl. 313/204, 313/492 [51] Int. Cl. H0lj 61/10 [58] Field ofSearch 313/204, 109, 193, 195

[56] References Cited 9 UNITED STATES PATENTS Lemmers 313/204 X Swanson313/204 Larson et al. 313/109 Primary E.\aminerHcrman Karl SaalbachAssistant Examiner-Siegfried H. Grimm Attorney, Agent, or FirmJohn T.OHalloran; Menotti J. Lombardi, Jr.; Edward Goldberg [57] ABSTRACT A-low pressure metal vapor discharge lamp, such as of mercury vapor, isprovided with internal interdigitated apertured baffle plates. Thislengthens the discharge path and reduces temperature to provideincreased efficiency and improved operation at higher intensities. Thebaffles may be of insulation or conductor materials and may be coatedwith a protective layer, fluorescent material or ultra-violet reflectivelayer.

8 Claims, 4 Drawing Figures PATENIE, rm 1 21974 SHEET 2 OF 2 DISCHARGELAMP WITH 'BAFFLE PLATES BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to electric illumination sourcessuch as metal-vapor discharge lamps of lowpressure mercury-vapor and, inparticular, those lamps which have a plurality of baffle plates betweendischarge electrodes for optimizing tube efficiency.

2. Description of the Prior Art In the illumination field, low-pressuremercury-vapor discharge lamps are used as efficient high intensitysources capable of supporting high electric or thermal loads. For theselamps, light efficiency depends not only on the quantity of electricalenergy received, but also on the efficiency of resonant radiationexcited in the metal vapor. This last factor is preponderant as far asthe source excitation is concerned. The most advantageous value forresonant radiation with mercury vapor corresponds to a rather lowconstant mercury vapor pressure, as is well known to those skilled inthe art. This results in a number of drawbacks when energy received bythe source is increased so as to increase illumination thereof. Inparticular, temperature increases as well as vapor pressure, andresonant radiation efficiency decreases. The increase in illumination isno longer proportional to the quantity of electrical energy suppliedwhich limits the output for such lamps.

Various remedies have been attemped. Thermal dissipation has beenincreased by enlarging the envelope surface with the same length byproviding sinuousshaped glass lamps with baffles. Several types ofbaffled glass envelopes have produced improved efficiency. Inparticular, envelope shapes presenting a cross-section with reentrant orgrooved portions are of advantage in increasing illumination efficiency,lamps utilizing mercury vapor having a resonant radiation of 2,537 A. to

excite the luminescent material coating the internal mercury-vapor lampglass-envelope wall. As a result, higher light emissions per unit lengthhave been produced with a given illumination efficiency for largerelectrical loads.

These results may be explained by considering an example of alowpressure mercury-vapor discharge lamp having such a baffle structurewhich provides an electron velocity increase, a reduction of energylosses caused by elastic collisions, and an electron and mercury iondiffusion improvement resulting in an improved radiation emission rateat the. wave length of 2,537 A. at the internal envelope walls. For agiven length and electrical power per unit length, the lamp utilizes arelatively low current and a voltage which is higher than correspondingvalues for same circumference circular-cross-section lamps which resultsin a reduction of losses in the cathode and ballast. In addition, it iseasier to hold the mercury vapor pressure at the optimum value toenhance emission efficiency at the particular radiation of 2,537 A. Theangles joining the envelope wall on both sides of the baffle regionreentrant portions are colder than the rest of the tube. Theirtemperature increases as the load increases, but not as rapidly as therest of the cross-section. This is due to plasma or dischargeconstriction which directs the discreases to reduce the local heatingeffect.

However, such arrangements do not permit satisfactory adjustment of themercury vapor pressure which is naturally determined by the coldestpoint of the envelope. To overcome this and to produce illuminationradiation of high stability, it has been suggested that shields beplaced behind the cathodes to provide a cold point at each discharge endof the lamp. This solution has the drawback of requiring additionalspace in the lamp that does not provide illumination radiation and asymmetrical structure at each end which results in a substantial loss ofuseful length.

Special glass envelope designs have been used wherein a specific contourpermits substantial cancellation of the discharge within a given regionso as to provide a cold point for adjusting pressure in the usefulportion. This has the advantage of not lengthening the lamp whileimproving the adjustment, but the amount of control is still notsufficient. Other complex shapes of glass envelopes with baffles orreentrant portions and folds in the reentrant portions for cancellingthe discharge result in a more sophisticated and costly product.Accordingly, such solutions have not been utilized. For the purposes ofcontrolling mercury vapor pressure in these lamps, another satisfactorysolution has been found by inserting a movable amalgam in the glassenvelope, particularly an indium amalgam such as described in FrenchPat. No. 1,583,078 in name of the present applicant.

SUMMARY OF THE INVENTION It is therefore the primary object of thepresent invention to provide an improved discharge lamp which utilizesbaffles inside the envelope between electrodes to lengthen the dischargepath. The baffles cooperate with the internal envelope wall to providegreater illumination flux per unit length of the lamp. The glassenvelope may have a conventional shape such as cylindrical.

According to another feature of this invention, the lengthened dischargepath is accomplished by a plurality of serially-associatedinterdigitated baffles, which may be prefabricated independently fromthe envelope itself. This avoids the drawbacks of sophisticated as wellas costly glass envelope shapes. The plurality of baffles are associatedwith the internal glass envelope wall and may be made of one or severalinsulator materials which include amorphous silicates of the glassfamily, such a fiberglass or crystallized silicates such as mica.

According to another feature of this invention, the material of theplurality of baffles enclosed inside the glass envelope may be of anelectrical conductor material, such as aluminum, nickel, iron, etc. Insome embodiments, the material of the baffles may have a protectivecoating and in one specific embodiment, the said coating is of amaterial reflecting ultra-violet radiations. In another case, thecoating is fluorescent. In a further embodiment, the discharge tubeincludes an indium amalgam which may be mobile within the tube.

In addition, the invention includes a process for manufacturing suchdischarge lamps which comprises the following steps:

a. forming a plurality of baffles having an external pattern ofidentical shape but slightly smaller than the internal pattern of theenvelope cross-section,

b. inserting and mounting the interdigitated baffles in the envelope,

c. evacuating and degassing the lamp, and

d. filling the lamp with a metal vapor and closing and sealing the lamp.

Other features of this invention will become apparent from the followingdescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is a schematic axialcross-sectional view of a mercur'y'vapor discharge lamp according to afirst embodiment of this invention,

FIG. lb is a transverse cross-sectional view of the baffle arrangementinside the lamp shown in FIG. la,

FIG. 2a is a schematic axial cross-sectional view of a second embodimentof a mercury-vapor discharge lamp wherein the baffle arrangementprovides a sinusoidal path for the discharge through the tube, and

FIG. 2b is a schematic perspective view of the baffle arrangement shownin FIG. 2a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the first embodimentillustrated in FIGS. la and lb, the metal vapor discharge lamp 1comprises a cylin drical glass envelope 2, wherein discharge occursbetween internal electrodes 3 and 4 at opposite ends. According to thisinvention, a set of interdigitated baffles 5 is positioned betweenelectrodes 3 and 4. FIG. 1b shows this arrangement in a cross-sectionalview taken along line AA of FIG. 1a normal to the tube axis. Theassembly of baffles 5 includes an axially extending plate 6 having afull length slightly shorter than the distance between the endelectrodes and a laterally extending width slightly shorter than or atmost equal to the internal tube diameter. When the plate 6 is mountedinside the tube, it divides the interelectrode space into two regions,an upper region and a lower region, connected by apertures 9 in theplate 6. Plate 6 is secured on a series of interdigitatedhalf-circular-shaped baffle-plates 7, 8 extending from opposite sides ofthe envelope nor mal to plate 6. The baffle plates have radius slightlyshorter than internal radius of envelope 2. Along a direction normal tothe plane of plate 6, an upper baffleplate 7 is followed by a lowerbaffle-plate 8 which is in turn followed by another baffle-plate 7, andso on. Each aperture 9 in plate 6 is positioned between each pair ofsuccessive adjacent baffle plates 7, 8.

The entire assembly of baffles 5 is prefabricated in a simple mannercompletely independently of the discharge lamp envelope. For example,the assembly has been made of a suitable metal such as nickel oraluminum. Plate 6 is a sheet of small thickness and apertures such as 9are easily produced by piercing, for example. Baffle-plates 7 and 8 arefastened to plate 6 either by direct welding or by any other suitablemethod. Since overall sizes are slightly smaller than the internal glassenvelope, it is simple to position such a baffle assembly inside theenvelope by threading and sliding it along the walls during themanufacturing step when only one tube end is closed. The lamp assemblyis then completed, dried and evacuated, filled with gas and sealed atthe end in the usual manner.

Operation is illustrated in FIG. la which shows a dashed line path 10-10for the discharge between electrodes 3 and 4. Since the discharge isforced to follow the predetermined path due to the interdigitatedbaffles and apertures, the discharge path is lengthened geometricallyfor a particular direct distance between electrodes. Such an arrangementmakes it possible to substantially increase power per unit of lengthwithout degrading the electrical characteristics of the lamp powersupply as in previous designs.

The baffle arrangement may be embodied either in a metal sheet as in theabove described example or in an insulating material such as fiberglass.In both cases, the material is so selected that it supports drying andvacuum degassing, which are basic for proper assembly of the lamp.

Various other materials which support drying and vacuum degassing havebeen tried in manufacturing these lamps and very good results have beenobtained independent of the electric conduction quality of the material.This permits a much larger selection of materials than if use waslimited to insulator materials. In addition, various coating materiallayers have been successfully applied. For example, the baffle plate 8has been coated with an inactive protective-type layer 11 such ascadmium and anodized aluminum.

A baffle arrangement has also been made of a mate rial coated with anactive fluorescent-type layer by applying a fluorescent powder similarto that coating the internal glass wall. This resulted in substantialimprovement in illumination efficiency since the excited fluorescentsurface is substantially increased. Lastly, an active ultra-violetreflecting layer, such as titanium oxide, has also been tried. Asubstantial increase of the discharge lamp illumination efficiency wasobtained since the absorbed radiation portion is reduced to a minimum.

In a further embodiment, a certain amount of indium was introduced inaddition to the usual mercury dose. The same controlled mercury pressurewas obtained as when the tube did not include the baffle assembly. Theindium may be held on a resilient support as described in the abovementioned French Pat. No. 1,583,078. The position of the amalgam duringlamp operation is a function of temperature. In this manner, cumulativeadvantages are obtained from both the use of amalgam for controllingpressure and the use of baffles for lengthening the discharge path.

In all the above mentioned examples, the power sup ply is provided in aconventional manner by a selfinductance, a leakage autotransformer, or acurrent limiting device. In all these cases the discharge is establishedin a satisfactory manner using the baffles. The electricalcharacteristics are very different from those provided by a lamp havingthe same length, but without baffles. Voltage per unit length (volt/cm)for the same current intensity, depends primarily on the geometricarrangement of the baffles. The greater the number of baffles, thehigher the lamp performance, since the baffles further lengthen thedischarge path.

In this respect, various other geometric baffle arrangements may be usedto obtain longer and more complex meander paths. The improvement isdirectly proportional to the lengthening of the geometric path. FIGS. 2aand 2b illustrate a more sophisticated geomet' ric baffle arrangementthan those of FIGS. Ia and 1b. In such an arrangement, the assembly 25includes oblique baffle-plates such as 27 associated with base plate 26provided with apertures 29, 29' symmetrically located with respect tothe opposite outer sides of plate 26 within the boundaries 30 providedby upper and lower oppositely angled baffle plates.

Assembly 25 is positioned within the lamp as in the first embodiment bysliding it along the inner wall of envelope 2. The line 31 of FIG. 2aillustrates the pattern of the discharge path resulting from baffleassembly 25. The discharge path is substantially lengthened. The path 31has a sinusoidal shape. Elementary geometric considerations show thatthe discharge path is approximately four times longer than the distancebetween electrodes. Electrical measurements also show that voltagegradients, in voltage per unit length (volt/cm), are multiplied by four.The actual improvement is proportional to the lengthening of thedischarge path. These embodiments are not limited to the use of mercuryvapor lamps, the same advantages being obtained with sodium, cadmium andother discharge spaces connected through apertures in said plate, and aplurality of lateral baffle plates supported in said envelope betweensaid electrodes forming a lengthened meandering discharge path throughsaid vapor.

2. The metal vapor lamp of claim 1 wherein successive lateral baffleplates are secured alternately on opposite upper and lower sides of saidlongitudinal plate, respective apertures being positioned between pairsof adjacent alternate plates.

3. The metal-vapor lamp of claim 2 wherein said lat eral bafflc platesare at oblique angles with respect to the longitudinal axis of said lampforming a sinusoidal discharge path.

4. The metal-vapor discharge lamp of claim 2, wherein said baffle platesare of an insulator material.

5. The metal-vapor discharge lamp of claim 2, wherein said baffle platesare of an electrical conductor material.

6. The metal-vapor discharge lamp of claim 2, wherein said baffle platesinclude a protective coating layer.

7. The metal-vapor discharge lamp of claim 2, wherein said baffle platesinclude an ultra-violet reflecting layer.

8. The metal-vapor discharge lamp of claim 2 wherein said baffle platesinclude a fluorescent layer.

1. A metal-vapor discharge lamp comprising a longitudinal envelopehaving a metal-vapor therein, electrodes at opposite ends of saidenvelope, a longitudinal baffle plate dividing said envelope into upperand lower spaces connected through apertures in said plate, and aplurality of lateral baffle plates supported in said envelope betweensaid electrodes forming a lengthened meandering discharge path throughsaid vapor.
 2. The metal vapor lamp of claim 1 wherein successivelateral baffle plates are secured alternately on opposite upper andlower sides of said longitudinal plate, respective apertures beingpositioned between pairs of adjacent alternate plates.
 3. Themetal-vapor lamp of claim 2 wherein said lateral baffle plates are atoblique angles with respect to the longitudinal axis of said lampforming a sinusoidal discharge path.
 4. The metal-vapor discharge lampof claim 2, wherein said baffle plates are of an insulator material. 5.The metal-vapor discharge lamp of claim 2, wherein said baffle platesare of an electrical conductor material.
 6. The metal-vapor dischargelamp of claim 2, wherein said baffle plates include a protective coatinglayer.
 7. The metal-vapor discharge lamp of claim 2, wherein said baffleplates include an ultra-violet reflecting layer.
 8. The metal-vapordischarge lamp of claim 2 wherein said baffle plates include afluorescent layer.